Fluxgate Magnetometer - Sensing magnetic fields

Certain materials undergo sudden, complete saturation when a well defined amount of current flows (high magnetic permeability materials).  If these materials are used as cores, and if a large enough alternating current is driven through the coil, then the magnetic polarity alternates along with the driving current through a cycle of magnetised - unmagnetised - inversely magnetised - unmagnetised – magnetised.

The above cycle of flipping magnetic polarity occurs symmetrically (with opposing saturation occurring on each half of the driven cycle).  However, in the presence of an external magnetic field along the axis of the core, the symmetry is altered.  Since saturation is dependant on total magnetic intensity (external + internal fields), then the  part of the cycle that generates a field in the same sense as the external field (a stronger field) will saturate sooner, and the part generating a field in the opposite sense will saturate later (a weaker field).  

It is this asymmetry which is measured and is the basis for the fluxgate magnetometer.